pinctrl: rockchip: rk3188: Fix support for IOMUX_GPIO_ONLY flag

GPIO0_A0-A7 on RK3188 is IOMUX_GPIO_ONLY, however, trying to set gpio
mux return an -ENOTSUPP error code. Fix this by validating using the mux
function type and not the iomux flag.

Based on Linux commit c4a532dee6b6 ("pinctrl: rockchip: handle first
half of rk3188-bank0 correctly").

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>